I oroginally logged a cal about 3 weeks ago, but then I contacted Dell support via email saying that the problem was getting worse. We got a phone call from one of the Dell support teams in India saying that the engineer would call to swap the drives.
I don't know if it makes any difference but the machine was bought via my company.
I ended up having to re-install the disks myself as the Dell engineer installed them as RAID 0 (something that I can't understand!) so I ended up deleting that array, then putting one of the original (failing) disks back in the machine on port 0. The machine booted up found that disk, and hence the original array, but with an unassigned disk (one of the new ones) this I assigned to the array and then booted into Vista from the original disk, so no settings where lost, and let the software get on with rebuilding the array.
Then I shut down the machine, took the original disk out, and put the other new one in, and repeated the process, so I ended up with the 2 new disks in the machine but with the original install - so I didn't have to re-install any software.
The actual physical installation of the disks is a quick 2 minute job, and the array re-build took about 1 to 2 hours each time - but at least I could walk away and let it get on with it rather than having to re-install and setup everything again.
BTW What is happening to your old drives, are Dell going to collect them off you?
Hope the above helps.